For a drug to be effective, a certain concentration level (called the therapeutic index) must be maintained for a certain period of time, at specific location(s). Systemically administered drugs accomplish the first two objectives, but in an inefficient fashion and with the potential for toxic side effects at high doses. Systemic administration of controlled release formulations accomplish these two objectives with a more efficient utilization of the drug and may reduce side effects. Local implantation of drug delivery systems may further improve the efficiency of drug utilization.
Hydrogels are materials that absorb solvents (such as water), undergo rapid swelling without discernible dissolution, and maintain three-dimensional networks capable of reversible deformation. Hydrogels may be uncrosslinked or crosslinked. Uncrosslinked hydrogels are able to absorb water but do not dissolve due to the presence of hydrophobic and hydrophilic regions. Covalently crosslinked networks of hydrophilic polymers, including water soluble polymers, are traditionally denoted as hydrogels in the hydrated state. A number of aqueous hydrogels have been used in various biomedical applications, such as, for example, soft contact lenses, wound management, and drug delivery.
Hydrogels can be formed from natural polymers such as glycosaminoglycans and polysaccharides, proteins, etc., where the term “glycosaminoglycan” encompasses complex polysaccharides that are not biologically active (i.e., not compounds such as ligands or proteins) and have repeating units of either the same saccharide subunit or two different saccharide subunits. Some examples of glycosaminoglycans include dermatan sulfate, hyaluronic acid, the chondroitin sulfates, chitin, heparin, keratin sulfate, keratosulfate, and derivatives thereof.
Glycosaminoglycans may be extracted from a natural source, purified and derivatized, or synthetically produced or synthesized by modified microorganisms such as bacteria. These materials may also be modified synthetically from a naturally soluble state to a partially soluble or water swellable or hydrogel state. This can be done, for example, by conjugation or replacement of ionizable or hydrogen bondable functional groups such as carboxyl and/or hydroxyl or amine groups with other more hydrophobic groups.
Hydrophilic polymeric materials suitable for use in forming hydrogels include poly(hydroxyalkyl methacrylate), poly(electrolyte complexes), poly(vinylacetate) cross-linked with hydrolyzable bonds, water-swellable N-vinyl lactams polysaccharides, natural gum, agar, agarose, sodium alginate, carrageenan, fucoidan, furcellaran, laminaran, hypnea, eucheuma, gum arabic, gum ghatti, gum karaya, gum tragacanth, locust beam gum, arabinogalactan, pectin, amylopectin, gelatin, hydrophilic colloids such as carboxymethyl cellulose gum or alginate gum crosslinked with a polyol such as propylene glycol, and the like. Several formulations of previously known hydrogels are described in U.S. Pat. No. 3,640,741 to Etes, U.S. Pat. No. 3,865,108 to Hartop, U.S. Pat. No. 3,992,562 to Denzinger et al., U.S. Pat. No. 4,002,173 to Manning et al., U.S. Pat. No. 4,014,335 to Arnold, U.S. Pat. No. 4,207,893 to Michaels, and in Handbook of Common Polymers, (Scott and Roff, Eds.) Chemical Rubber Company, Cleveland, Ohio.
Synthesis and biomedical and pharmaceutical applications of absorbable or biodegradable hydrogels based on covalently crosslinked networks, comprising polypeptide or polyester components as the enzymatically or hydrolytically labile components, respectively, have been described by a number of researchers. See, e.g., Jarrett, et al., “Bioabsorbable Hydrogel Tissue Barrier: In Situ Gelation Kinetics”, Trans. Soc. Biomater., Vol. XVIII, 182, 1995 and Park, “Enzyme-digestible Swelling Hydrogels as Platforms for Long-term Oral Drug Delivery: Synthesis and Characterization”, Biomaterials, 9:435 (1988).
The hydrogels most often cited in the literature are those made of water soluble polymers, such as polyvinyl pyrrolidone, which have been crosslinked with naturally derived biodegradable components such as those based on albumin. Totally synthetic hydrogels that have been studied for controlled drug release, and as membranes for the treatment of post-surgical adhesion, are based on covalent networks formed by the addition polymerization of acrylic-terminated, water soluble chains of polyether dipolylactide block copolymers.
Bioabsorbable hydrogels are well suited for local implantation, but relatively low molecular weight molecules are rapidly released from hydrogels due to the relatively open networks of previously known hydrogels. Relatively low molecular weight compounds, however, constitute a vast majority of therapeutic molecules and drugs. Controlled drug delivery from implantable and bioabsorbable devices has been the subject of extensive exploration, but no suitable absorbable systems are known that are capable of delivering both water soluble and water insoluble relatively low molecular weight drugs.
The development of compositions and methods to provide controlled release delivery of relatively low molecule weight drugs presents the following challenges: the delivery matrix needs to be safe and absorbable; drug release should be controlled and sustained, while being free from “burst effects”; and the devices should be simple to fabricate so as to prevent denaturation of sensitive entrapped drugs.
Previously known methods and compositions for providing sustained controlled release of therapeutic species, and applications suitable for use of such compositions and methods, are discussed hereinbelow, and include: (a) microencapsulation and (b) targeted microspheres.
a. Microencapsulation
Several previously known delivery systems employ biodegradable microspheres and/or microcapsules that include biodegradable polymers, such as poly d,l-lactic acid (PLA) and copolymers of lactic acid and glycolic acid (PLGA). These polymers are most widely used in sustained release devices, and may be obtained by polycondensation of lactic acid or glycolic acid in the presence or absence of a catalyst or other activator. Microcapsules prepared from such materials may be administered intramuscularly or by other parenteral routes.
The water solubility of a number of biologically active molecular compounds, however, has proven to be a limiting factor in optimizing molecular compound loading efficiency in biodegradable microspheres and/or microcapsules. Specifically, it has been observed that the loading efficiency of water soluble drugs into, for example, PLA or PLGA-polymeric microspheres, is relatively low when conventional oil/water systems are used in a solvent evaporation process. This has been attributed to the observation that such drugs readily diffuse into the aqueous outer phase of the emulsion system.
Most of the microspheres described in the literature belong to the class of “matrix-type” drug delivery capsules, in which the “foreign” (i.e. drug) particles are dispersed homogeneously in direct contact with the polymer. The process of manufacturing such capsules also frequently involves direct contact between the drug and a polymer solvent, such as acetonitrile or methylene chloride. Such contact between the biologically active molecule and the polymer, polymer solvent or enzymes in the biological system may promote degradation of the intended pharmaceutical.
Specifically, the monomer and dimer residues in the polymer may degrade the protein, and direct contact between the polymer and proteins and enzymes may result in polymeric degradation over time. Previously known techniques to encapsulate peptides in biodegradable polymers typically utilize a solvent-nonsolvent system. Such systems often produce high solvent residuals, poor content uniformity of the peptide in the microspheres, and instability due to the contact of the biological agent with the polymer, organic solvent (e.g. methylene chloride, acetonitrile), and some cases, a surfactant.
To address the use of organic solvents that may have a potentially detrimental effect on entrapped substances, and which complicate processing, several alternate methods have been proposed. U.S. Pat. No. 5,589,194 to Tsuei et al. describes preparation of microcapsules by dispersing or dissolving an active component in a solid matrix-forming material that has been thermally softened to form an encapsulation composition. The encapsulation composition is injected as an intact stream into a quenching liquid to provide solid microcapsules.
U.S. Pat. No. 3,242,237 to Belak et al. describes a process for forming discrete slow release fertilizer particles, wherein solid fertilizer is dispersed in melted wax and dropped into water in the form of droplets. The droplets solidify in particle form upon contact with the water, and are separated from the water.
European Patent application 0 443 743 to Kubota discloses a method to encapsulate particulate Vitamin C in fine lipid powders by ringing a particulate core containing Vitamin C into colliding contact with particles of a coating material composed on one or more fine powdery lipids. The lipids form a coating of agglomerated particles that surround the particulate core.
U.S. Pat. No. 3,161,602 to Herbig discloses a process for making capsules utilizing a three-phase system: a wax-like wall material, a nucleus material, and a substantially inert oily vehicle. The waxy material is melted to a liquid and agitated to coat the nucleus material, forming liquid-walled capsule precursor droplets. The solution is cooled with continued agitation, solidifying the waxy walls and forming self-sustaining capsules.
The process described in the foregoing Herbig patent has a number of drawbacks, however, including an undesirably long time span from the formation of liquid droplets to the completely solid capsules (which may cause loss of the active component either via diffusion or exclusion mechanisms into the hot inert oily vehicle); it requires high mechanical agitation; may produce capsules having an uneven distribution of active ingredient; and may produce capsules having a very broad size distribution.
U.S. Pat. Nos. 4,597,970 and 4,828,857, both to Sharma et al., describe a method to encapsulate aspartame in hydrogenated palm oil using a spray drying process. That process has disadvantages shared with other air spray processes, however, in that it is difficult to provide a uniform, continuous layer on the outermost surface of the droplets during the congealing step.
U.S. Pat. No. 3,423,489, to Arens et al. and U.S. Pat. No. 3,779,942 to Bolles describe methods of forming capsules by forming concentric biliquid columns having an inner core of liquid to be encapsulated and an outer tube of hardenable liquid encapsulating material. A special multiple orifice liquid discharging system is used to eject the column along a trajectory path through, e.g., a gaseous phase, for a time sufficient to allow the column to constrict into individual droplets, so that the encapsulating material encloses the encapsulated liquid.
Torchilin et al, “Liposome-Polymer Systems. Introduction of Liposomes into a Polymer Gel and Preparation of the Polymer Gel inside a Liposome”, in Polymer. Sci. U.S.S.R., 30:2307-2312 (1988), describes studies on the entrapment of liposomal particles in non-absorbable hydrogels. Liposomes may be difficult to prepare and stabilize. Also, the non-absorbable nature of polyacrylamide hydrogels precludes implantation without subsequent retrieval. As reported by Bailey et al., “Synthesis of Polymerized Vesicles with Hydrolyzable Linkages”, Macromolecules, 25:3-11 (1992), while synthesis of polymerizable liposome vesicles also has been attempted, the complicated synthesis scheme makes entrapment of drug molecules difficult in this process.
U.S. Pat. No. 5,618,563 to Berde et al. describes use of a polymeric matrix, including microspheres, to release analgesic agents locally at the site of implantation. The polymer matrix used in that patent is not a hydrogel, and hydrophobic polymers are used for entrapment of the drugs. Such polymer matrices, however, may be inflammatory.
U.S. Pat. No. 4,530,840 to Tice et al. describes a method for forming microcapsules to deliver an anti-inflammatory agent. The microcapsules are prepared by dissolving the anti-inflammatory agent and a biodegradable wall-forming material in a solvent, and then dispersing the resulting solution in a continuous phase processing medium. The processing medium evaporates a portion of the solvent from the dispersion, thereby forming microparticles containing the anti-inflammatory agent. The organic solvents described in this method may damage some sensitive therapeutic entities, and residual solvents used in the process may be difficult to remove and present a toxicity concern.
U.S. Pat. No. 5,650,173 to Ramstack et al. reviews the state of the art of formation of microparticles suitable for encapsulating drugs and for providing controlled drug delivery. One method for preparing biodegradable microparticles is described that uses solvents to dissolve both the wall-forming agent and the drug. An extraction medium is used to remove the solvents and stabilize the resulting emulsion to form the microparticles. As with the methods described in the Tice patent, the use of organic solvents in large amounts may raise removal and toxicity issues.
In view of the foregoing, it would be desirable to provide compositions and methods for implementing a locally implantable and absorbable drug delivery system that is capable of delivering relatively low molecular weight compounds in a sustained fashion within hydrogel-based matrices that are easy to process and fabricate.
b. Targetable Microspheres
Numerous disease states in the body are manifested as local conditions and thus may be addressed by local therapies. In addition, local pain (such as from an incision) or solid tumors may be treated locally. Targeting of local therapy may be assisted by a host of non-invasive and invasive detection techniques such as magnetic resonance imaging, ultrasound, x-rays, angiography, etc.
Despite the availability of such diagnostic tools, however, the pinpointing of the location of a disease may at times be more difficult. This may be so either due to the diffuse nature of the disease or due to subtle alterations at the cellular or microscopic level that escape detection by conventional means, for example metastatic tumors or autoimmune disorders. Potent drugs with known efficacy exist for several such diseases, but too many of these drugs have undesirable toxicity profiles at therapeutic levels.
Efficient utilization of a drug by targeted delivery may enable reduction of concomitant toxicity. For example, microspheres for intravenous injectable drug delivery typically should be of a size so as to not to be rapidly cleared from the blood stream by the macrophages of the reticuloendothelial system. U.S. Pat. No. 5,565,215 to Gref et al. describes formation of injectable nanoparticles or microparticles that have variable release rates or that target specific cells or organs.
Liposomal drug delivery systems have been extensively considered for the intravenous administration of biologically active materials, because they were expected to freely circulate in the blood. It has been observed, however, that liposomes are quickly cleared from the blood by uptake through the reticuloendothelial system. Coating of liposomes with poly(ethylene glycol) has been observed to increase substantially the half life of such active materials. The flexible and relatively hydrophilic PEG chains, however, induce a steric effect at the surface of the liposome that reduces protein adsorption and thus RES uptake. See, e.g., Lasic et al., “Sterically Stabilized Liposomes: a Hypothesis on the Molecular Origin of the Extended Circulation Times”, Biochimica et Biophysica Acta, 1070:187-192 (1991); and Klibanov et al., “Activity of Amphipathic Poly(ethylene glycol) 5000 to Prolong the Circulation Time of Liposomes Depends on the Liposome Size and Is Unfavorable for Immunolipososome Binding to Target”, Biochimica et Biophysica Acta, 1062:142-148 (1991).
The field of immunology has enriched our understanding of cell surface receptors and signaling molecules. For example, most cell populations exhibit a unique set of receptors that makes it possible to create “monoclonal antibodies” that are cell population and target specific. Knowledge of this specificity has enabled the development of therapies such as those adopted by fusion toxins, that bind cytotoxic molecules (such as ricin) to monoclonal antibodies against specific receptors of a certain cell population (such as tumor cells). Such therapies generally have not been widely successful, however, for reasons that are not well understood. For example, there may be inadequate selectivity in targeting due to the brief exposure time afforded by intravascular administration of these soluble molecules prior to rapid clearance.
Approaches toward enhancing circulation time using immunoliposomes have been more successful in assimilation in the target organs of interest. Since liposomes are only a few nanometers in size, however, these materials have a much higher circulation velocity. See, e.g., Ley et al., “Endothelial, Not Hemodynamic, Differences Are Responsible for Preferential Leukocyte Rolling in Rat Mesenteric Venules”, Circ. Res., 69:1034-1041 (1991). This rapid circulation may interfere with the building of strong interactions with the target tissues by providing only limited exposure to the liposomes.
The adhesion of leukocytes in general, and monocytes in particular, to vascular endothelium is a crucial first step to the recruitment of cells from the blood to the site of tissue damage. Leukocytes do not simply circulate within blood vessels but rather experience a “rolling” type of motion along the vessel wall that allows them to interact with the endothelial cell lining. This rolling motion is believed to be caused by weak interactions mediated by carbohydrate molecules (called selectins) present on the cell surface.
Upon receiving an appropriate activation signal, the endothelial cells slow down (mediated by L and possibly P selectins), and subsequently form more firm attachments (usually mediated by protein-based receptors such as integrins), as reported in Raud et al., “Leukocyte Rolling and Firm Adhesion in the Microcirculation”, Gastroenterology, 104:310-323 (1993). This in turn causes a local accumulation of leukocytes and allows their participation in physiological processes such as inflammation and repair. Often this behavior is associated with vascular injuries associated with inflammatory conditions. For example, after cardiac bypass procedures, endothelial cells that become anoxic may change their selectin expression pattern and cause neutrophils to attack, thereby causing potentially life-threatening “reperfusion injury,” as reported in Edginton, “New Horizons for Stem-Cell Bioreactors”, Bio/Technology, 10:1099-1106 (1992).
Much of septic shock is mediated by similar mechanisms. The progression of several diseases, such as arthritis and cancer, may be altered by stopping leukocyte adherence, which is the first step to extravasation (movement into the tissue spaces). Much may be learned from how the body targets specific disease sites through receptor mediated guidance.
Accordingly, it would be desirable to provide compositions and methods that enhance the targetability of microencapsulated drug carriers, which may be readily prepared and administered, but are still highly specific in finding the target tissue and efficient in the delivery of the drug. Such “smart microspheres” may be able to achieve improved targeting by having lower circulation velocity, slower clearance from circulation, and by possessing selective adhesivity to selected cellular targets.